Method for producing graphene oxide with tunable gap

ABSTRACT

A method of fabricating a graphene oxide material in which oxidation is confined within the graphene layer and that possesses a desired band gap is provided. The method allows specific band gap values to be developed. Additionally, the use of masks is consistent with the method, so intricate configurations can be achieved. The resulting graphene oxide material is thus completely customizable and can be adapted to a plethora of useful engineering applications.

RELATED APPLICATIONS

This filing is a continuation application of U.S. patent applicationSer. No. 14/095,454, filed Dec. 3, 2013, which is a continuationapplication of U.S. patent application Ser. No. 13/316,771, filed Dec.12, 2011, now U.S. Pat. No. 8,609,458, which claims the benefit of andpriority to U.S. Provisional Patent Application No. 61/422,092, filedDec. 10, 2010, all of which are incorporated by reference herein intheir entireties and for all purposes.

FIELD OF THE INVENTION

The current invention is directed toward a method for producing asemiconducting graphene oxide material in which the oxidation isconfined to the graphene layer.

BACKGROUND OF THE INVENTION

Graphene, a material comprising a lattice of carbon atoms positioned ina ‘honeycomb-type’ arrangement and tightly joined by in-plane sp² bonds,has garnered much attention from research communities because its uniqueelectrical and mechanical properties make it potentially ideally suitedfor various engineering applications, such as nanoelectronics andintegrated circuits. Compared to other conductive and semiconductivematerials, graphene has a superior carrier mobility and low resistivity,making it a promising candidate for integrated circuits. (Castro Neto AH, et al., Rev Mod Phys. 2009, 81, 109-62; Geim A and Novoselov K.,Nature Materials. 2007, 6, 183-91; Novoselov K S, et al., Science. 2004,306, 666-9; Novoselov K S, et al., Nature. 2005, 438, 197-200; NovoselovK S; McCann E, et al. Nat Phys. 2006, 2, 177-80; and Zhang Y, et al.,Nature. 2005, 438, 201-4, the disclosures of which are incorporatedherein by reference.)

However, graphene is inherently a semimetallic material—as opposed to asemiconductor material—and this limits its usability. (See, e.g.,Oostinga J B, et al., Nature Materials. 2007, 7, 151-7; Ni Z H, et al.,ACS nano. 2008, 2, 2301-5; Pereira V M, et al., Physical Review B. 2009,80, 045401; Han M Y, et al., Phys Rev Lett. 2007, 98, 206805-8; NakadaK, et al., Phys Rev B. 1996, 54, 17954-61; and Ponomarenko L A, et al.,Science. 2008, 320, 356-8, the disclosures of which are incorporatedherein by reference.) As a result, researchers have employed a number ofmethods to introduce a finite band gap within graphene, and therebyconvert it into a semiconductor. One approach to introduce an energy gapopening in graphene is to break its lattice symmetry using foreign atomssuch as hydrogen, gold, nitrogen, oxygen, and organic molecular dopants.(See, e.g., Bostwick A, et al., Physical Review Letters. 2009, 103,056404; Balog R, et al., Nat Mater. 2010, 9, 315-9; Sessi P, et al.,Nano letters. 2009, 9, 4343-7; Geirz I, et al., Nano Lett. 2008, 8,4603-7; Wehling T, et al., Nano letters. 2008, 8, 173-7; Luo Z, et al.,Appl Phys Lett. 2009, 94, 111909-11; Leconte N, et al., ACS nano. 2010,4, 4033-8; Nourbakhsh A, et al., Nanotechnology. 2010, 21, 435203-11;Kim D C, et al., Nanotechnology. 2009, 20, 375703, Alzina F, et al.,Physical Review B. 2010, 82, 075422 Gokus T, et al., ACS nano. 2009, 3,3963-8; Childres I, et al., New Journal of Physics. 2011, 13, 025008;Dong X, et al., Small. 2009, 5, 1422-6; and Lu Y H, et al., The Journalof Physical Chemistry B. 2008, 113, 2-5, the disclosures of which areincorporated herein by reference.)

For example, researchers have used wet oxidation methods to insertforeign atoms into the graphene structure. These impurities alter thesp² carbon hybridization in graphene to a sp³ carbon hybridization, andeliminate the π-bonds that facilitate charge transport across thegraphene plane. Consequently, with diminished charge transport thedesired band gap is obtained. (See, e.g., Elias D C, et al., Science.2009, 323, 610-3; Sofo J O, et al., Physical Review B. 2007, 75, 153401;and Boukhvalov D W, et al., Physical Review B. 2008, 77, 035427, thedisclosures of which are incorporated herein by reference.) However, thewet oxidation process is less than ideal: it typically uses harshchemicals, such as strong acids and oxidizing agents; it takes asignificantly long time to complete; and it does not allow for thecreation of site specific oxidation, which substantially limits theusability of this modified graphene. (See, e.g., Hummers W S and OffemanR E, J Am Chem Soc. 1958, 80, 1339; Park S and Ruoff R, Naturenanotechnology. 2009, 4, 217-24; Li D, et al., Nat Nanotechnol. 2008, 3,101-5; Sun X, et al., Nano Res. 2008, 1, 203-12; Becerril H A, et al.,Nano Lett. 2008, 2, 463-70, the disclosures of which are incorporatedherein by reference.)

Researchers have also experimented with using plasma oxidation, a dryoxidation method, to create a graphene oxide semiconductor material.This method is advantageous in a number of respects: it does not use anyharmful chemicals; it is a more rapid process; and it allows for sitespecific oxidation. For example, Nourbakhsh et al. have fabricated andcharacterized such a graphene oxide layer. (See Bandgap Opening inOxygen Plasma-Treated Graphene, Nourbakhsh et al. Nanotechnology. 2010,21, 435203-11, the disclosure of which is incorporated herein byreference.)

Previous studies also show that the p-doping level, electron-electronscattering rate, and the total density of states of an UV/ozone treatedgraphene are dictated by the defect density associated with surfaceconcentration of oxygenated functional groups and oxygen molecule. At avery low defect density, the p-doping level and electron-electronscattering rate increase in proportion to the increase in defectdensity. At a higher defect density, a continuous decay and smoothing ofthe van Hove singularities becomes apparent, and a further increase inthe defect density results in a significant drop in the conductance.This indicates a strong Anderson metal-insulator transition, with anoverall change in the carrier concentration in the order of 10¹² cm⁻².These studies also show that an increase in defect density becomesincreasingly difficult as the oxygen adsorption reaches a constant valueafter a certain UV/ozone exposure time. (See, e.g., Leconte N, et al.,ACS nano. 2010, 4, 4033-8; Nourbakhsh A, et al., Nanotechnology. 2010,21, 435203-11; Kim D C, et al., Nanotechnology. 2009, 20, 375703, AlzinaF, et al., Physical Review B. 2010, disclosed above.)

Similar electronic transport behaviors are also observed in oxygenplasma treated graphene, where the p-doping level increases with theincrease of oxygen plasma exposure, rendering the oxidized grapheneunipolar. As the oxygen plasma exposure increases further, the level ofdisorder in the structural symmetry of graphene becomes more pronounced,which leads to a decrease in conductance and mobility, as well as atransition from semimetallic to semiconducting behavior. However, theprior art has yet to develop a process for the production of asemiconductor graphene oxide material suitable for practicalapplications. For example, although Nourbakhsh et al. discusscharacterizing a graphene oxide layer created by a plasma oxidationprocess, the authors do not provide any guidance on how to avoid thecreation of oxides on the substrate surface. For example, the authorsincorrectly suggest that the band gap that can be created using this dryoxidation process can be as high as 3.6 eV (they reached this figure viacalculation). It has now been discovered that such high band-gaps areimpossible absent the destruction of the graphene oxide band gap.Accordingly a need exists for improved fabrication processes capable offorming graphene oxide materials in which the oxidation is confinedwithin the graphene layer such that they can be used in practicalapplications.

SUMMARY OF THE INVENTION

The present invention is directed to a novel fabrication method thatallows for a versatile but precise manipulation of graphene so as todevelop graphene oxide material that possesses an appreciable, anddeterminable, band gap, and in which the oxidation is confined to thegraphene layer. This novel fabrication method can thus develop grapheneoxides that are optimized for practical use. The process includessubjecting a graphene sample to a dry oxidation process for apre-determined period of time, wherein the length of oxidation exposuredetermines the magnitude of the band gap.

In one embodiment of the invention, masks are used in conjunction withthe oxidation process, and the masking and oxidation steps areoptionally iterated in order to achieve a desired configuration.

In yet another embodiment of the invention, a UV/Ozone oxidationprocess, which can allow for a greater control in achieving a desiredband gap, is used as the oxidation process.

In still another embodiment, a remote indirect plasma oxidationtreatment, which allows for more expedient oxidation while providing asafer oxidation treatment as compared to a direct plasma oxidationtreatment, is used as the oxidation process.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will be more fully understood when considered with respect tothe following detailed description, appended claims, and accompanyingdrawings, wherein:

FIG. 1 provides a flow chart of an exemplary embodiment of a process forforming a graphene oxide material in accordance with the currentinvention.

FIG. 2 provides a data plot showing exemplary experimental resultsdemonstrating how the band gap in a graphene oxide grows over time afterexposure to oxidation.

FIG. 3 provides data plots showing exemplary experimental resultsdemonstrating the band gap in a graphene oxide after exposure to (1) aUV/Ozone oxidation treatment, and (2) a plasma oxidation treatment.

FIG. 4 provides a data plot showing exemplary experimental resultscorrelating the size of the band gap with the oxygen concentration.

FIG. 5 provides a flow chart of an exemplary embodiment of a process forusing a masking technique in conjunction with the process for forminggraphene oxide material in accordance with the current invention.

FIG. 6 provides an optical microscopy image of partially oxidizedgraphene layer after 10 seconds oxidation at RF power of 20 watt, thecovered region exhibits energy gap opening of ˜0.1 eV (top left), whilethe uncovered region exhibits energy gap of ˜0.4 eV (bottom right), aphysical mask is used to partially cover a graphene layer from beingexposed to oxygen plasma or UV/ozone treatment (top right).

FIG. 7 provides data plots illustrating: (a) UV/ozone and oxygen plasmatreatments are employed to create an energy gap opening in graphenelayer, (b) Current-image of a pristine graphene layer obtained byscanning tunneling microscope (STM) showing a highly-symmetric hexagonallattice structure, (c) Raman spectrum of a pristine graphene layer wherethe peak intensity ratios of ID/IG and IG/IG′ are measured to be 0.09and 0.20 respectively (A single Lorentzian profile of the G′ band showsthe signature of a monolayer graphene).

FIG. 8 provides data plots illustrating: (a) typical averageddifferential conductance curves dI/dV of oxygen plasma and UV/ozonetreated graphene as probed using STS at more than five random locationsof the sample with stabilization voltage and current of 100 mV and 650pA respectively, (b) Energy gap opening in graphene as a function ofexposure time of oxygen plasma and UV/ozone treatments, (c) Lowresolution x-ray photoelectron spectroscopy (XPS) of graphene samples atdifferent degrees of oxidation, and (d) Energy gap opening in grapheneas a function of the oxygen concentration (oxygen-to-carbon atomicratio) of the samples (The oxygen-to-carbon atomic ratio (O/C ratio) isobtained from (c)).

FIG. 9 provides data plots illustrating: (a) typical high resolution C1s XPS spectra of (a) pristine, (b) UVO5m, (c) OP10s, (d) UVO120m, (e)OP60s samples, where deconvolution of these spectra usingGaussian-Lorentzian lineshape and Shirley baseline correction show thepresence of C—O, C═O, and O—C═O functional groups, and (f) surfaceconcentration of C—O, C═O, and O—C═O functional groups as a function ofthe energy gap in the LDOS of graphene samples.

FIG. 10 provides data plots illustrating typical high resolution O 1sXPS spectra of: (a) pristine, (b) OP10s, (c) UVO120m, (d) O2P60s, (e)UVO240m, and (f) OP150s samples, where deconvolution of these spectrausing Gaussian-Lorentzian lineshape and Shirley baseline correction showthe presence of C—O, C═O, and O—C═O functional groups, and where thepresence of a strong additional O 1s peak in (c) and (d) may beassociated with physisorption of oxygen.

FIG. 11 provides data plots illustrating typical high resolution Ni 2pXPS spectra of: (a) pristine, (b) UVO120m, (c) O2P60s, and (d) OP150ssamples, where deconvolution of these spectra using a combination ofLorentzian asymmetric and Gaussian-Lorentzian lineshape, as well asShirley baseline correction show the presence of a metallic Ni in allgraphene samples, and where the presence of a multiplet associated withNi(OH)₂ can be seen in the heavily oxidized graphene samples (b) and(c).

FIG. 12 shows atomically resolved STM images of (a) UVO5m, (b) O2P10s,(c) UVO120m, and (d) O2P60s for samples obtained at a scan rate of 20.3Hz and a stabilization voltage and current of 100 mV and 650 pArespectively, where the insets show Fourier transformed unit cells.

DETAILED DESCRIPTION OF THE INVENTION

The current invention is directed to a precise, versatile, and novel dryoxidation method of fabricating a semiconducting graphene material thatcan be used in a number of practical applications, such as:nanoelectronics, high frequency low noise field effect transistors whichcan be used for amplifiers, full wave rectifiers, RF resonators andswitches, and integrated circuits. In particular, the current inventionrecognizes that absent very rigorous process parameters, graphene oxidelayers are prone to the development of substrate oxides that cannegatively impact the electronic characteristics and usability of thematerials for practical applications, and has further discovered that byconfining the oxidation to the graphene layer it is possible to preventthese substrate oxides.

FIG. 1 provides a flowchart of a fabrication process in accordance withsome exemplary embodiments of the invention. As shown, in someembodiments, the fabrication process includes: 1) obtaining a suitablesubstrate; 2) depositing a graphene layer onto said substrate; and 3)subjecting the graphene layer to a dry oxidation treatment, wherein thedry oxidation treatment is confined to the graphene layer to prevent thedevelopment of oxide interaction with the substrate. In particular, aswill be described in greater detail below, the current inventionprovides methods of controlling the flux of oxidation and overallconcentration of oxide in the graphene surface to ensure that oxidationis confined within the graphene layer, and more particularly, such thatoxygen ions do not penetrate through the graphene to form oxides on theunderlying substrate thereby resulting in the creation of unwantedsubstrate oxides.

The following sections will elaborate on these basic fabrication steps,and will also provide descriptions of alternative embodiments that maybe used in accordance with the above fabrication steps:

Substrate Layer

A variety of substrates may typically be used in the fabrication ofelectronic devices. As described above, the current invention isdirected to graphene oxide materials that may be used in practicalelectronic applications. Accordingly, any substrate suitable for use asa structural foundation for a practical electronic device may be usedwith the current invention. For example, some commonly used substratematerials include: silicon, silicon dioxide, aluminum oxide, sapphire,germanium, gallium arsenide, an alloy of silicon and germanium, andindium phosphide, and it should be understood that any of thesesubstrate materials may be used with the fabrication process of thecurrent invention. In making a selection of an appropriate substratematerial in accordance with the current invention, it will be understoodthat it is preferable that the influence of the processing applicationson the substrate be minimized. In some cases this can be difficult,because the substrate is typically bonded to the electrical device priorto various processing applications, and so they too are subjected tothose processing applications. In particular, as discussed above and infollowing sections, it is important to the creation of high qualitygraphene oxide materials that the dry oxidation treatment be confined tothe graphene layer. Accordingly, in some embodiments the substratematerial can be chosen such that the material is resistant to theproduction of surface oxides from exposure to oxygen ions.

Graphene Layer

As described above, once a substrate material has been chosen, agraphene layer must be deposited thereon. With regard to this step ofthe fabrication process, it will be understood that the deposition ofgraphene can be achieved in any manner suitable for the formation of acontinuous graphene layer on the chosen substrate.

For example, in one embodiment of the invention, graphene samples can begrown by chemical vapor deposition techniques on nickel coated SiO₂/Sisubstrates at 900° C. under a flow of 25 sccm methane and 1500 sccmhydrogen precursor gases. The sample can then be exposed tovacuum-pyrolysis treatment at an elevated temperature of 250° C. and amild vacuum at 2.5 torr for 24 hours to remove the residual oxygenadsorbed during the growth process. Alternatively, in another embodimentof the invention, the graphene substrate configuration can be achievedby depositing single-layer graphene (SLG) flakes by micromechanicalexfoliation on n-doped Si substrates covered with a 90 nm thermallygrown SiO₂ film.

It should be understood that the above processes are merely provided asexamples and simply represent possible embodiments of the invention thatillustrate how the desired substrate-graphene configuration can beachieved. The enumeration of these embodiments is not meant to implythat they represent the only ways a substrate-graphene configuration canbe achieved in accordance with this invention—any method of obtainingthis graphene substrate configuration can be used consistent with theinvention.

Dry Oxidation and Variability of the Band Gap

As previously described with regard to the flow chart in FIG. 1, in someembodiments of the invention the graphene is exposed to a dry oxidationtreatment. Dry oxidation techniques (as opposed to wet oxidationtechniques) oxidize via a gas phase process and confer the followingbenefits: they typically do not require the use of any harsh chemicals;they are relatively quick procedures; and they are compatible with theuse of masks. FIG. 2 provides a data plot showing how the band gap in agraphene oxide grows over time after exposure to both plasma andUV/ozone dry oxidation processes. In this respect, any dry oxidationtechnique suitable for the controlled deposition of oxygen ions on asurface may be used with the current invention, including, for example,direct plasma oxidation treatment, indirect remote plasma oxidationtreatment, and UV/Ozone treatment.

Importantly, any of the above mentioned dry oxidation techniques wouldallow for the creation of specific desired band gaps within the graphenematerial to be achieved by controlling the oxidation process. Generallyspeaking, a longer exposure to the oxidation process will result in alarger band gap. The longer exposure time is understood to allow the dryoxidation treatment to induce oxygen adsorbates onto the graphene layer.These oxygen adsorbates introduce defects in the graphene's inherent sp²structure, and thereby disrupt the graphene's inherent if-bond network(the if-bond network is what facilitates the electron mobility andcharge transport across the graphene plane). FIG. 3 correlates the bandgap with the concentration of oxygen. Unaltered graphene has an oxygenconcentration of roughly 9%, whereas graphene with a band gap of roughly2.5 eV has an oxygen concentration of roughly 21%. Thus, measuring theoxygen concentration can help verify the presence of a band gap, and bycontrolling the concentration of oxygen it is possible to engineer theband gap of the graphene oxide to be between 0 and 2.5 eV.

Moreover, the specific type of dry oxidation treatment used also impactsthe variability of the band gap—the use of a plasma oxidation treatmentincreases the band gap much more rapidly than does the use of a UV/Ozoneoxidation treatment. This difference in rapidity is thought to be afunction of the different concentrations of reactive oxygen species perunit time present in both treatments. FIG. 4 illustrates the difference,and shows that a 60 second plasma oxidation treatment yields a 2.5 eVband gap, while a 2 hour UV/Ozone treatment yields just a 2 eV band gap.Therefore, using the UV/Ozone oxidation treatment allows the band gap tobe controlled with greater resolution. Conversely, using either of theplasma oxidation techniques allows a desired band gap to be achievedmuch more rapidly.

Although a number of different dry oxidation processes may be used withthe current invention to produce graphene oxides, and to engineer theband gap characteristics of these materials, with regard to the currentinvention it is of critical importance that the oxidation treatmentshould be confined to the graphene layer, and that, in turn, thesubstrate should not be exposed to the oxidation process. Exposure ofthe substrate to oxidation, and or leaching of the oxide from thegraphene layer to the substrate can lead to the formation of oxides onthe substrate, which can negatively impact the graphene oxide material.Accordingly, by controlling the process parameters to ensure that theoxidation is confined within the graphene layer it is possible toimprove surface quality and produce substantially defect-free grapheneoxide semiconductor material suitable for practical use.

In view of this, the inventive oxidation process is tailored to avoidthe formation of substrate oxides and ensure confinement of the oxidewithin the graphene layer.

-   -   First, in some such embodiments, the concentration of the oxide        is carefully monitored and controlled to ensure that it does not        exceed a maximum concentration of 21%. It has been determined        that oxygen concentrations of greater than 21% result in the        creation of substrate oxides, and the subsequent formation of        oxides with the substrate surface.    -   Second, in some embodiments an indirect remote plasma oxidation        treatment is used. Such an indirect remote plasma treatment is        less vigorous, and therefore reduces the possibility of damage        to the graphene surface, and/or the likelihood that oxygen ions        reach the substrate surface.

With the above process restrictions in mind, the following embodimentsof the invention are provided as examples and illustrate how dryoxidation treatments can be applied to a graphene sample in accordancewith the invention:

(1) In one embodiment of the invention, a UV/Ozone treatment is appliedat standard room temperature and pressure for a selected period of timesuch that the concentration of the oxygen within the graphene oxidelayer does not exceed 21%.

(2) In another embodiment of the invention, a remote oxygen plasmamachine (for example, the Tepla M4L) is used under 20 Watts of RF powerat a constant oxygen flow rate of 20 sccm and chamber pressure of 500mTorr for an appropriate amount of time. Note that in no case shouldmore than 50 Watts of RF power be used when using the plasma oxidationtreatment method. Again, the oxygen content within the graphene layermust be confined to 21% or less.

It should be understood that these are simply embodiments of theinvention that illustrate how dry oxidation treatments may be applied,and are not meant to limit the scope of the invention—any suitable dryoxidation treatment method may be used in conjunction with theinvention.

Masking

Although the above discussion has described basic embodiments of theinvention, in some alternative embodiments the fabrication process isused in conjunction with masking techniques to allow for the creation ofgraphene/graphene oxide materials with multi-function surfaces and/orvariable band gap regions. Masking is a technique used in circuitmanufacture that allows for the creation of multiple regions withdistinct electrical properties within a single layer. Essentially, masksare employed prior to the material's treatment, thereby protecting thecovered region from the treatment's influence.

Some such embodiments, a summary of which is shown in the flow chartprovided in FIG. 5, include 1) obtaining a suitable substrate; 2)depositing a graphene layer onto said substrate; 3) masking portions ofthe graphene layer; 4) subjecting the graphene layer to a dry oxidationtreatment, wherein the dry oxidation treatment is confined to thegraphene layer; and 5) optionally reiterating steps 3 and 4 if a moreintricate electronic configuration is desired.

Thus, in one embodiment of this invention, masking can be employed priorto the dry oxidation treatment. Any region protected by the mask duringan initial dry oxidation treatment will retain graphene's inherentsemimetallic properties. Using this technique, a graphene oxide materialwith multiple regions can be developed. Accordingly, it is possible toincorporate masking techniques with the current fabrication techniqueallows for the creation of more intricate graphene samples.

As shown in FIG. 6, in order to achieve site specific oxidation, aphysical mask is used to partially cover the graphene layer, preventingthe covered region from being exposed to oxygen plasma or UV/ozonetreatment. The unexposed region retains its gapless electronicproperties, while the energy gap of the exposed region starts to open updepending on the exposure time and power. For instance, in FIG. 6, after10 seconds of oxidation using oxygen plasma treatment at RF power of 20watt, the uncovered region exhibits an energy gap of ˜0.4 eV, while thecovered region can still be considered gapless.

Moreover, the masking and oxidation processes can be iterated to achieveeven further intricate semiconductor patterns. For example, a maskinglayer can be employed during an initial oxidation process, and thenremoved during a second oxidation cycle. The resulting material wouldhave two regions with different band gaps: the region that was subjectto the initial mask could have some appreciable band gap, whereas theregion that was not exposed to the masking but still subjected to bothoxidation treatments would have and even greater band gap. Moreover,different masking patterns can be used between the multiple oxidationsteps to achieve even further intricate patterns. Thus, in yet anotherembodiment of this invention, multiple masking and oxidation cycles areused to obtain multiple regions of varying band gaps.

Many applications will be made possible by having the ability to oxidizegraphene layer at a particular location or with a specific pattern.These applications include graphene based 2D LEDs, high frequencytransistors and solar cells. These devices will certainly take advantageof graphene's ballistic electron mobility behavior as well as itsintrinsic strong and light weight properties.

EXEMPLARY EMBODIMENTS

The following embodiments are only exemplary and illustrative in nature,and are not meant to limit the scope of the invention.

Materials and Methods

Graphene samples used in the following studies were grown by chemicalvapor deposition technique on nickel coated SiO₂/Si substrates at 900°C. under a flow of 25 sccm methane and 1500 sccm hydrogen precursorgases. (See, Reina A, et al., Nano Lett. 2009, 9, 30-5; Brien M O andNichols B, Sensors Peterborough NH. 2010, TR, 5047; and Reina A, et al.,Nano Research. 2009, 2, 509-16, the disclosures of which areincorporated herein by reference.) These as-grown samples were thenexposed to vacuum-pyrolysis treatment at an elevated temperature of 250°C. and a mild vacuum at 2.5 torr for 24 hours to remove the residualoxygen adsorbed during the growth process. In the following discussion,these vacuum-pyrolysis treated graphene samples are referred as pristinesamples. The presence of monolayer graphene on the pristine samples wasconfirmed by Raman spectroscopy (Renishaw M1000) obtained withexcitation energy of 2.41 eV.

Two different oxidation processes were applied to the pristine graphenesamples. The first set of graphene samples were oxidized by UV/ozonetreatment (Bioforce Nanosciences) at standard room temperature andpressure for 5 minutes, 30 minutes and 120 minutes. Another set ofgraphene samples were oxidized by remote oxygen plasma (Tepla M4L) under20 Watts of RF power at a constant oxygen flow rate of 20 SCCM andchamber pressure of 500 mTorr for 5 seconds, 10 second, 30 seconds and60 seconds. For brevity, in the discussion that follows, oxygen plasmatreated samples are referred as O2P samples followed by the exposuretime in seconds, and UV/ozone treated samples are referred as UVOsamples followed by the exposure time in minutes.

Atomic-resolution images of treated graphene samples were obtained usinga scanning tunneling microscope (Digital Instrument Nanoscope IIIaECSTM) equipped with Pt/Ir scanning tip (Veeco, Inc) at constant heightmode. During the imaging process, a graphene sample was placed on a flatsample stage and clamped from the top with a metal electrode thatcreates a direct contact with the graphene film. Tunneling I-V anddifferential conductance characteristics were obtained using thetunneling spectroscopy capability of the same scanning tunnelingmicroscope. All scanning tunneling microscopy/spectroscopy (STM/STS)imaging and measurements were conducted at room pressure and temperatureat a scan rate of 20.3 Hz and a stabilization voltage and current of 100mV and 650 pA respectively. Differential conductance characteristic,dl/dV, of each sample was obtained by averaging dl/dV curves from matleast five randomly selected locations on the sample. At higher degreesof oxidation, these curves were obtained from regions that still exhibitreasonable atomic periodicity and can be imaged without excessive noiseusing STM. Surface chemistry characterizations were assessed using x-rayphotoelectron spectroscopy (Surface Science M-Probe XPS). The XPSspectral analysis was performed using a Gaussian-Lorentzian curve-fitwith Shirley baseline correction.

Example 1 Band Gap Studies

The atomically resolved image of pristine graphene samples obtained byscanning tunneling microscope (STM) exhibits a highly-symmetrichexagonal lattice structures, which is a typical signature of a pristinegraphene layer. In agreement with previously reported study, thesehexagonal lattice structures show an atomic spacing of ˜0.23 nm (FIG. 7b). (See, e.g., Mizes H A, et al., Physical Review B. 1987, 36, 4491, thedisclosure of which is incorporated herein by reference.) Three distinctpeaks of D band (˜1350 cm-1), G band (˜1580 cm-1) and G′ band (˜2700)can be seen in the Raman spectrum of the pristine graphene samples (FIG.7c ). (See, e.g., Dresselhaus M S, et al., Nano Lett. 2010, 10, 751-8;Reina A, et al., Nano Lett. 2009, 9, 30-5; and Dresselhaus M S, et al.,Annual Review of Condensed Matter Physics. 2010, 1, 89-108, thedisclosure of which is incorporated herein by reference.) The peakintensity ratio between the disorder-induced D band and sp² symmetry Gband, ID/IG, was measured to be 0.09. The presence of monolayer graphenecan be deduced from the existence of a strong single Lorentzian profileof G′ band with an intensity ratio IG/IG′ of 0.20. (See, Dresselhaus M,et al., Philosophical transactions—Royal Society Mathematical, Physicaland engineering sciences. 2008, 366, 231-6, the disclosure of which isincorporated herein by reference.)

As revealed by scanning tunneling spectroscopy (STS), the tunneling I-Vcharacteristics of the oxidized graphene samples exhibit a deviationfrom that of the pristine graphene samples around the zero-bias region,where a sign of tunneling current flattening start to occur once thegraphene is oxidized. As graphene undergoes longer duration ofoxidation, this flat region becomes wider and more apparent. Forexample, the lightly oxidized UVO5m and O2P5s graphene show a vague flatregion of 0.2 eV and 0.3 eV, while the heavily oxidized UVO120m andO2P60s graphene show a flat region as large as 1.8 eV and 2.4 eV. Thislarge deviation of I-V characteristics of the oxidized graphene to theas-grown one suggests a strong correlation between the oxidation processand the electronic characteristics of oxidized graphene.

The evolution of electronic characteristics of oxygenated graphene canbe understood by investigating the tunneling differential conductance,which is proportional to the local density of states (LDOS), at variousdurations and types of oxidation process. The tunneling differentialconductance presented herein was calculated numerically by taking thefirst derivative of the tunneling current with respect to the biasvoltage (FIG. 8a ). As expected, the tunneling differential conductanceof the pristine graphene samples shows that their LDOS is zero atzero-energy, which confirms that their Fermi level is zero at the Diracpoint. Another feature that is noticeable in the tunneling differentialconductance curve is the presence of two peaks surrounding thezero-energy, which may be attributed to the constructive interference inphonon-mediated inelastic scattering. (See, Brar V W, et al., PhysicalReview Letters. 2010, 104, 036805; and Rutter G M, et al., Science.2007, 317, 219-22, the disclosure of which is incorporated herein byreference.)

In contrast to that of the pristine graphene samples, the tunnelingdifferential conduction spectra of the oxidized graphene samples shows asign of flattening around the zero-energy region, suggesting aconsiderable suppression in the LDOS around the zero-energy. The mildlyoxidized UVO5m graphene show a narrow flat region of about 0.2 eV aroundthe zero-energy region. The suppression in the LDOS becomes much morepronounced as the graphene samples undergo a prolonged oxidation time.In fact, the heavily oxidized UVO120m and O2P60s graphene show extendedsuppression in the LDOS up to 1.8 eV and 2.4 eV (FIG. 8a ). Theoccurrence of such energy gap in the LDOS suggests that the electroniccharacteristic of oxidized graphene has been transformed from zeroenergy gap semimetallic, into semiconducting or even insulator. (See,e.g., Leconte & Nourbakhsh, disclosed above.)

In agreement to the previous studies, the extent of the energy gap ofoxidized graphene seems to depend heavily on the oxidation time, wherelonger exposure time to UV/ozone and oxygen plasma treatments results inlarger energy gap opening. (See, Alzina, Gokus & Childres, cited above.)It is important to note that the increase of energy gap opening inoxygen plasma treated graphene is significantly faster than that inUV/ozone treated graphene. For instance, after only 60 seconds of oxygenplasma treatment, the O2P60s graphene has an energy gap of 2.44 eV. Incontrast, 120 minutes of UV/ozone treatment gives the UVO120m graphenean energy gap of 1.93 eV. Such difference may be caused by differentconcentrations of reactive oxygen species per unit time present in bothtreatments. Note that in both treatments the energy gap opening does notincrease linearly with the increase of oxidation time (FIG. 8b ). Infact, a further opening of energy gap becomes increasingly difficult asthe oxygen adsorption reaches saturation very rapidly, and defects arecreated on the surface that may render the graphene oxide unsuitable foruse in electronic application. (Leconte, cited above.)

It can be expected that the opening of energy gap in the LDOS ofoxidized graphene is induced by the introduction of disordered defectsin the sp² structure of graphene due to the presence of oxygenadsorbates. These defects produce a strong disruption to the π-bondnetwork that facilitates the electron mobility and charge transportacross the graphene plane. (Elias, cited above.) Since the magnitude ofsuch disruption depends heavily on the spatial distribution of defectssites and the degree of induced localization, an increase of defectdensity will certainly reduce the electron mobility. (See, Bostwick &Luo, cited above.) In addition, any alteration to the π-bond networknear the defect sites further distorts the electron-phonon couplings andelectron-electron interactions. (See, Luo, cited above; and Manes J L,Physical Review B. 2007, 76, 045430, the disclosure of which areincorporated herein by reference.) Therefore, the energy gap in the LDOSobserved in this study can be associated with the electron mobility gapintroduced by disordered defects in the .pi.-bond network. (See,Childres, cited above.)

A more meaningful relation can be obtained by correlating the energy gapof the oxidized graphene to its oxygen-to-carbon atomic ratio (O/Cratio). Basically, the O/C ratio represents the surface concentration ofoxygen adsorbates. In this study, the O/C ratio of graphene samples wasobtained from the x-ray photoelectron spectroscopy (XPS) survey scans.Multiple peaks related to C 1s, O 1s, Si 2s and Ni 2p can be seen on theXPS spectra of all graphene samples (FIG. 8c ). Because the intensity ofphotoelectron is directly proportional to the atomic density of thesample, the fractional atomic concentration of oxygen and carbon atomscan be inferred from the intensity of the O 1s and C 1s peaks. (See,Hesse R, et al, Surface and Interface Analysis. 2005, 37, 589-607; andPeng Y and Liu H, Industrial & Engineering Chemistry Research. 2006, 45,6483-8, the disclosures of which are incorporated herein by reference.Notice that the intensity of the O 1s peak increases as the graphenesamples undergo a prolonged oxidation process. Also note that theintensity of O 1s peak of pristine graphene is not zero, which suggeststhat a small amount of oxygen is readily adsorbed at the grapheneboundaries during the growth or storage phase and may not be easilyremoved.

As expected, the correlation between energy gap of an oxidized grapheneand its O/C ratio is monotonic, where a higher O/C ratio yields a largerenergy gap, regardless of the oxidation method used. This findingimplies that the observed energy gap opening is indeed introduced bydefects created by oxygen adsorbates, which create disruption in the.pi.-bond network. These defects also induce a localization effect,where each of the oxidized site acts as a strongly repulsive hard wallbarrier, and the degree of such localization is dictated by the spatialdistribution and the density of the oxidized sites. (See, Luo, citedabove.) In addition, the correlation confirms the existence of an O/Cratio threshold around 15%, above which the energy gap opening increasesexponentially (FIG. 8d ). Such threshold might be explained as acrossing from weak to strong localization regimes in graphene aslocalization quickly spreads over all energy spectrum at the very strongdisorder regime.

In contrast to the previous studies, a slight increase in O/C ratiobelow this threshold does not increase the opening of energy gapdramatically. (See, Leconte & Kim, cited above.) At such regime,however, the experimental data seem to agree with the prediction done byelectronic band structure calculation, where an O/C ratio of ˜12% yieldsan energy gap opening of ˜0.2 eV. (See, Nourbakhsh, cited above.) On theother hand, a slight increase in O/C ratio above this threshold resultsin drastic increase of energy gap opening, which is not quite inagreement with the prediction done by electronic band structurecalculation. Experimental data show that an energy gap opening of ˜1.5eV can be obtained by an O/C ratio of just ˜18%. Clearly, suchprediction underestimates the energy gap opening because the electronicgap calculation is only valid for graphene that retains its structuralintegrity. At a high O/C ratio (greater than 21%), the defect densitybecomes extremely high such that it is unlikely that the band structurehas survived. This implies that the observed opening of energy gapbeyond this limit may not be a band gap, and therefore is not suitablefor use in electronics applications.

Example 2 XPS Studies

The presence of oxygen adsorbates in the oxidized graphene samples wasfurther investigated using high-resolution XPS scans. Curve-fitting anddeconvolution of the high-resolution XPS spectra of C 1s was performedusing a Gaussian-Lorentzian peak shape with Shirley baseline correction.Deconvolution of the C 1s XPS spectra of both oxygen plasma and UV/ozonetreated samples shows four distinct peaks associated with sp² C—C(284.7±0.1 eV, FWHM 0.9 eV), C—O (285.2±0.1 eV, FWHM 1.45 eV), C═O(286.7±0.1 eV, FWHM 1.45 eV), and O—C═O (288.6±0.1 eV, FWHM 4 eV). (See,Yang D, et al., Carbon. 2009, 47, 145-52, the disclosure of which isincorporated herein by reference.) The C 1s XPS spectra of the pristinegraphene samples show a very strong C—0 peak which may be caused by asignificant presence of hydroxyl or epoxide groups at the edge. Thesespectra also show a relatively weak C═O peak and the absence of a peakassociated with O—C═O group (FIG. 9a ). The O—C═O peak can be barelyseen in the C 1s XPS spectra of lightly oxidized samples, i.e. O2P5s,UVO5m, O2P10s, and O2P10s samples (FIG. 9b and FIG. 9c ). A morepronounced O—C═O peak can be seen in the C 1s XPS spectra of the heavilyoxidized samples, i.e. UVO120m, O2P30s, and O2P60s samples (FIG. 9d andFIG. 9e ), suggesting a significant presence of carboxyl groups.

As mentioned earlier, the energy gap opening of graphene samples can becorrelated to their oxygen adsorbates concentration. The pristinegraphene samples are expected to have a very low surface concentrationof oxygenated functional groups. On the other hand, higher surfaceconcentration of these groups is expected to be found in graphenesamples that have larger energy gap. Although the correlation is notexactly linear, the surface concentration of these groups does increaseas the increase of energy gap opening of the graphene samples (FIG. 9f). The surface concentration of C—O group increases significantly from˜10% to ˜35% as the energy gap increases from 0 eV to 2.4 eV. Noticethat a large increase in the concentration of the C—O group is needed toinitiate the energy gap opening of the graphene. Similarly, the surfaceconcentration of C═O and O—C═O groups increases from ˜3% to ˜8% and ˜0%to ˜12% respectively for the same increase of energy gap. This findingimplies that majority of oxygen adsorbates introduced by oxygen plasmaor UV/ozone treatment is in the form of hydroxyl and carboxyl groups.

An oxygen uptake by the graphene layer during the oxidation process canalso be observed in the O 1s and Ni2p XPS spectra of both oxygen plasmaand UV/ozone treated samples. The O 1s spectra show three distinctcomponents associated with O—C═O (531.5±0.3 eV, FWHM 1.4 eV), C═O(532.6±0.3 eV, FWHM 1.3 eV), and C—O (533.5±0.3 eV, FWHM 1.4 eV). (See,Yang, cited above.) The area percentage of each O 1s spectral componentagrees with the corresponding component in the C 1s spectra. A major O1s peak, which can be seen obviously in UVO120m and O2P60s samples, maybe associated with absorbed hydroxide species or water (535.2±0.7 eV,FWHM 1.9 eV). (See, Biesinger M, et al., Surface and Interface Analysis.2009, 41, 324-32, the disclosure of which is incorporated herein byreference.) An additional weak O 1s peak (536.2±0.8 eV, FWHM 2.1 eV) maybe associated with physisorption of oxygen. (See, Biesinger, citedabove.) The main Ni metal 2p_(3/2) spectral component can be found at532.5±0.5 eV with FWHM of 1 eV, while the second and third ones can befound at binding energy shifts of +3.65 eV and +6.05 eV respectively,with FWHM of 2.6 eV and 2.8 eV respectively.

Example 3 Substrate Oxide Studies

It is important to note that the energy gap opening reported herein isnot induced by the production of nickel oxide or hydroxide during theoxidation process. High-resolution O 1s XPS spectra show the nonexistence of peaks associated with Ni—O for all samples, even after 60seconds of oxygen plasma and 120 minutes of UV/ozone treatments (FIG.10a-d in the ESM). A distinct Ni—O peak can only be seen in the O 1sspectra of samples that have been oxidized even further, e.g. afterbeing oxidized for 150 seconds in oxygen plasma or 240 minutes inUV/ozone treatments, (FIG. 10e-f in the ESM). Additional evidenceprovided by the Ni 2p_(3/2) spectra shows that the nickel catalyst layeris not oxidized, and remains in metal form in most of the samples.However, a weak presence of Ni(OH)₂ can be observed from the Ni2p_(3/2)spectral component of the heavily oxidized samples, i.e. UVO120m,O2P30s, and O2P60s samples (FIG. 11 a-c in the ESM). A significantfootprint of Ni(OH)₂ can only be observed in the Ni 2p_(3/2) spectra ofsamples that have been oxidized even further, e.g. after being oxidizedfor 150 seconds in oxygen plasma treatment (FIG. 11d in the ESM).

It will be understood from the findings mentioned above that the oxygenuptake is indeed caused by the oxidation of graphene, as long as thesamples are not over oxidized. For the heavily oxidized samples, i.e.UVO120m, O2P30s, and O2P60s samples, a small fraction of oxygen uptakeis caused by the production of Ni(OH)₂. Such an uptake may also inducean energy gap in the LDOS, rendering the data invalid for energy gaplarger than ˜1.9 eV. If the energy gap data from these heavily oxidizedsamples are omitted, one can find an almost linear relation between theenergy gap opening and the overall surface concentration of theoxygenated functional groups.

Example 3 STM Studies

The effect of both oxygen plasma and UV/ozone treatments to theelectronic structure of graphene can be literally seen from theevolution of the atomically resolved images obtained by scanningtunneling microscope (STM). As graphene is oxidized, defects on thehexagonal lattice structure due to the presence of oxygen adsorbates inthe form of oxygenated functional groups start to occur. For sampleswith low concentration of oxygen adsorbates, i.e. O2P5s and UVO5msamples, the degree of disorder is quite low such that the hexagonallattice structures still can be recognized from the raw STM imageswithout using any further image processing technique (FIG. 12a ). Athigher concentration of oxygen adsorbates, the distortion to the latticestructure is amplified such that the hexagonal patterns become much lessapparent and more difficult to be recognized from the raw STM images.Fourier transformation of the raw STM images of O2P10s and UVO30msamples reveals a superposition of a hexagonal lattice structure and arectangular lattice structure with a size of ˜0.41 nm by ˜0.25 nm.Previous study suggests that this rectangular unit cell, which isindependent of scan speed and azimuthal scanning direction, representsthe abundance of oxygenated functional groups, in particular thehydroxyl and carboxyl groups, on the oxidized graphene. (See, e.g.,Pandey D, et al., Surface Science. 2008, 602, 1607-13; and BuchsteinerA, et al., J Phys Chem B. 2006, 110, 22328-38, the disclosures of whichare incorporated herein by reference.)

At even higher concentration of oxygen adsorbates, i.e. UVO120m andO2P60s samples, the defect density becomes very high, such that thehexagonal patterns cannot be recognized anymore from the raw STM images(FIG. 12c and FIG. 12d ). Fourier transformation of the raw STM imagesof O2P60s samples shows a very faint hexagonal lattice structuresuperposed on a more intense rectangular lattice structure, suggesting astrong manifestation of oxygenated functional groups, in particular thehydroxyl and carboxyl groups, on its surface. (See, Pandey, citedabove.) Such high surface concentration may induce extra strain to thealready perturbed lattice structure which initiates lattice structurebreaking. This might explain why the STM images of the highly oxidizedgraphene samples appear more disordered and chaotic; the concentrationof carboxyl groups has increased from 0% in the pristine graphene to˜12.2% in the O2P60s samples. Judging from the existence of thesefunctional groups, one may expect that the highly oxidized graphenesamples exhibit different electronic characteristics to their pristinecounterparts.

CONCLUSION

Applicant has disclosed a novel fabrication method that allows for aversatile but precise manipulation of graphene so as to develop withinit an appreciable band gap while at the same time ensuring thatoxidation is confined within the graphene layer. Such a graphenematerial can be very well-suited for a host of practical electronicapplications.

The experimental findings presented herein show the effect of dryoxidation processes, in particular oxygen plasma and UV/ozonetreatments, to introduce an energy gap opening in graphene. The openingof the gap itself can be correlated to the surface concentration ofoxygen adsorbates, where the energy gap increases strongly as theincrease of oxygen adsorbates concentration. In fact, an increase ofoxygen-to-carbon atomic ratio from ˜9% to ˜21% is enough to increase theenergy gap opening from 0 eV to ˜2.4 eV. Note that a significantlyobservable energy gap opening occurs when the oxygen adsorbatesconcentration is higher than the oxygen-to-carbon atomic ratio thresholdof ˜15%. At high oxygen adsorbates concentration (˜21%), the defectdensity becomes extremely high such that it is unlikely that the bandstructure has survived. This implies that the observed opening of energygap may be associated with a mobility gap, not a band gap. The presenceof oxygenated functional groups, e.g. C—O, C═O and O—C═O groups, inducesdistortion to the .pi.-bond network of the graphene such that thehexagonal patterns become much less apparent and the rectangular unitcells become much more pronounced. In general, the oxygen plasmatreatment gives a much faster rate of oxidation than the UV/ozonetreatment. On the other hand, the slower oxidation rate of UV/ozonetreatment may provide a better control over the degree energy gapopening.

DOCTRINE OF EQUIVALENTS

Those skilled in the art will appreciate that the foregoing examples anddescriptions of various preferred embodiments of the present inventionare merely illustrative of the invention as a whole, and that variationsin the fabrication methodologies of the present invention, may be madewithin the spirit and scope of the invention. Accordingly, the presentinvention is not limited to the specific embodiments described hereinbut, rather, is defined by the scope of the appended claims. Unlessotherwise specified, all of the references disclosed herein areincorporated by reference into the specification.

1-22. (canceled)
 23. An sp2 structure graphene oxide material structure comprising: an sp2 structure graphene layer on a substrate, the sp2 structure graphene layer comprising defects and having a band gap based on the defects, and wherein an oxygen-to-carbon atomic ratio of oxidization of sp2 structure graphene layer is no greater than 21%, with oxidation is confined to the graphene layer.
 24. The structure of claim 23, wherein at least one portion of the sp2 structure graphene layer is not oxidized.
 25. The structure of claim 24, wherein the sp2 structure graphene layer includes a plurality of oxidized graphene portions, each of said portions having a desired band gap.
 26. The structure of claim 25, wherein each of the portions have different band gaps.
 27. The structure of claim 23, wherein the band gap is proportional to the concentration of oxidation within the sp2 structure graphene layer.
 28. The structure of claim 23, wherein the band gap ranges from 0.1 to 2.5 eV, and wherein the oxygen-to-carbon atomic ratio within the sp2 structure graphene is from about 9% to 21%.
 29. The structure of claim 23, wherein the substrate is a material selected from the group consisting of silicon, silicon dioxide, aluminum oxide, sapphire, germanium, gallium arsenide, an alloy of silicon and germanium, and indium phosphide. 